Ferromagnetic ferrite and process of preparing same



-- June 19,1962 3 v E. G. F ORTiN FERROMAGNETIC FERRITE AND PROCESS OF PREPARING SAME Filed Jan. 30. 196-1 INVENTOR.

EUBENE BFURTIH' V United States Patent Ofiice smassz Patented June l9, 1962 FERROMAGNETIC FERRITE AND PROCESS OF PREPARING SAME I Eugene G. Fortin, Hyde Park, Mass assignor to'Radio Corporation of America, a corporation of Delaware Filed Jan. 30, 1961, Ser. No. 85,524 12 Claims. (Cl. 252- 625) This invention' relatesto ferromagnetic ferrite bodies which exhibit a substantially square hysteresis loop and to methods of manufacture thereof. The bodies of the invention may be used as elements in coincident current memories of digital electronic computers,'and in other electronic apparatus. The terms core and body are used interchangeably in this document to refer to a sintered mass of ferrite material.

When a certain value of unidirectional magnetizing forcesuificiently greater than the so-called coercive force of the core is applied to a body or core which exhibits a square hysteresis loop, the core assumes one of two definite, identifiable stable states of remanent magnetization which are arbitrarily designated the zero and one states. The core can be changed or switched to the other stable, remanent state by applying thereto a magnetizing force of the same value in the reverse direction. Cores which require a relatively large magnetizing force for switching from one state to the other are referred to as high drive cores. Cores which require relatively small magnetizing force for switching from one state to th other are referred to as low drive cores. switches from one stable state to another, the change may be monitored by a read-out or sense winding around the When the core core. The change in state induces an output signal pulse in the sense winding. The magnitude of the output signal and the so-called signal to noise ratio should be as high as possible. I

One compositional family of ferrite cores which exhibit a substantially square hysteresis loop is magnesium-manganese ferrites containing up to a total of 8 percent of zinc oxide, cadmium oxide, or both zinc oxide and cadmium oxide. It was believed that cores of magnesium-manganese ferrites containing higher proportions than 8 percent of zinc oxide and/or cadmium oxide exhibit hysteresis loops with rounded corners and lower coercive force; and lower output signal voltage and signal to noise ratio.

It has now been found that cores within a limited compositional range of inagnesiurnmanganem ferrites containing both zinc oxide and cadmium oxide in total proportion greater than 10 mol percent unexpectedly exhibit square hysteresis loops and improved electrical properties.

An object of this invention is to provide improved ferromagnetic ferrite bodies.

Another object is to provide improved methods for manufacturing ferromagnetic ferrite bodies.

A further object is to provide improved ferromagnetic ferrite cores which exhibit substantially rectangular magnetic hysteresis loops and a relativelyhigh signal output upon switching.

In general, the improved bodies are formed by firing a mixture consisting essentially of magnesium, manganese, zinc, cadmium, and iron oxides in the proportions of about:

. The combined molar percent of ZnO, plus is at least 10 molpercent and the molar-proportion of ZnO is greater than the molar proportion of CdO.

The improved mixed ferrospinel bodies of the invention 2 are preferably prepared by first calcining in air at about 500 to 700 C. an intimate physical mixture consisting essentially of the foregoing composition. The calcine is compacted to a coherent body, then sintered at about 1100 to 1300 C. in air,- and finally annealed in a neutral atmosphere at about 1025 to 1100 C.

The invention is described in greater detail by reference to the accompanying drawing in which: 7

FIGURE 1 is a magnetic core of the invention in the shape of a toroid,

FIGURE 2 is a typical hysteresis loop for one of the mixed ferrospinel bodies herein, and

-FIGURE 3 is a magnetizing pulse program simulating operation of a typical ferromagnetic ferrite body as a memory core in a coincident current magnetic memory device. t

Exarhple.A ferromagnetic core of the invention may be prepared by the following procedure. Mix and calcine in air for one hour at about 525 C. the following composition: i-

. M01 percent MgO, as magnesium carbonate, Mallinkrodt SL powder 26 MnO, as MnCo Bakers analyzed reagent grade 22 ZnO, Bakers analyzed reagent grade 9 CdO, Bakers analyzed reagent grade '3 Fe O as-Mapico Red Fe O No. 110-2 40 The calcine is ground in water for about two hours, then dried and screened. The screened calcine is recalcined for about two hours in air at about 950 C. The recalcined batch is reground and about 3 weight percent of a suitable binder is added. The recalcined batch with binder added is screened through an 80 mesh screen. The screened recalcined batch is then pressed into toroids. The pressed toroids are then sintered for about 10 hours in air at about 1200 C. The sintered toroids are cooled to about 1060 C., annealed for about two hours in dry prepurified nitrogen, and then cooled to room temperature in the nitrogen atmosphere.

FIGURE 1 illustrates a toroidal core 21 prepared according to the example which comprises a shaped ferromagnetic core consisting essentially of sintered ferrite crystals. A typical fired toroid has about the following dimensions:

The characteristics of the core 21 of the example are tabulated in Table I as item 1.

Also shown in FIGURE 1 is an input winding 23 which' may be used for applying current to the core, for example, for test purposes. An output winding 27 is also shown. The input and output windings 23 and 27 are, in this instance, each a single wire passed through the orifice in the core 21. The operation of the core may be explained in terms of the hysteresis'loop shown in FIGURE 2, in which the ordinate is magnetization or flux densityB of the core and the abscissa is magnetizing force H applied to the core. Let the l3 flux state be defined as 1 and the +13 flux state be defined as 0. If a current pulse (1 of sufiicient amplitude and appropriate polarity is out winding 27. If another +H pulse of the same amplitude as the original+H pulse is applied to the core which is'now in its state, the core will produce only a small change in flux and, consequently, a low output voltage.

To be suitable for use incoincident-currentapplicat-ions, (and in certain others) a core must have magnetic properties such that, when the core is in either the 1 or the ,0 state, at remanence (no magnetizing force should occur only with the first few pulses (the less the better), after which no further change should occur.

This state is called the disturbed 1. Conversely, the application of pulses to a core in the +13 state produces a disturbed 0 state. v 1

Y A qualitative measure of the foregoing characteristics is the squareness ratio R which is defined as the ratio:

The current pulse program shown as curve 33 in FIG- URE 3 produces the following sequence of events: Pulse No. l of value +1; reads" an undisturbed 1 output Signal (uV which was written by pulse No. 11 of value l; of the previous cycle. When pulse No. l iscompleted, the core is left in its 0 state. Pulses No. 2 through No. 9 all of value 1g/2 are partial pulses (I which disturb the 0 state, but do not switch the core. Pulse No. 10 of value +1: returns the core to its undisturbed 0 state and reads the disturbed 0 output signal (a'V Pulse No. 11 switches the core and writes a 1. By omitting pulse No. 10 from the pro'grarn,-the flux change caused by pulse No. 11 produces a disturbed 1 (dV output signal. The coercive force H '(or H,,) is the magnetizing force at which the hysteresis loop intersects the magnetizing force axis. The foregoing criteria and their significance are understood in the art. For example, see the article by I. J. Sacco, ]r., in the RCA Engineer, vol. 5, No.' 4, pp. 16 et seq., December 1959, January 1960.

The following compositional and procedural variations may be made from that set forth iiithe example. The magnetic cores herein are within the following compositional ranges in mol percent:

MgO 10 to 30, preferably 20 to 30. MnO 20 to 25.

ZnO 5 to 25, preferably 5 to 14. CdO l to 12, preferably 1 to 7.5. Pa e, 35 to 42.

' ically pure grade of chemicals.

In the example, the steps of mixing, calcining at 525 C., grinding, drying, and screening are designed to provide an intimate mixture of the ingredients and for the removal of the gases, water, and inorganic matter. These steps are not critical. Any procedure which provides a dry, intimate mixture of the ingredients is satisfactory.

' pulses is applied, slight degradation of the B, state magnetic properties of the final product.

In the example, recalcining at about 900 C. is important. The recalcining temperature may be between 900 C. and 1100 C., 'butpreferably at the lower end of the range. The recalcining time is not critical, although shorter times should be used with higher recalcining temperatures. Air is the preferred recalcining atmosphere although atmospheres having oxidizing characteristics 'Similar to air at the recalcining temperature may also be used. I

In the example, r'egrinding the calcine, addition of a binder, rcscreening, and pressing are not critical to the However, a proper selection should be made to obtain the desired shape and size of product with a minimum of distortion. Besides toroids, other shapes such as magnetic memory plates and transfluxor cores may be prepared. See a description of fabrication processes in G. S. Hipskind and T. Q. Dziemianowicz, Processing and Testing Rectangular Loop Cores," RCA Engineer, volume 2, No. 6, 1957, pages 9 to 13.

In the example, the sintering temperature should be between 1100" C. and 1300 C. The zinc oxide volatilizes excessively above 1300- C. destroying the compositional proportions. Below 1100 C., the composition remains insufiic'iently reacted. The sintering time is not critical. Any sintering time suflicient for complete reaction is adequate. One to twenty hours, preferably ten hours, have been found to be a convenient firing time.

The sintering atmosphere is of great importance. In the example, the toroids are fired at about 1200 C. in air, and then annealed for two hours in dry prepurified nitrogen at about 1060 C. By thisprocedure the annealing atmosphere is critical since it determines the relative oxidation states of the constituents of the compositions. It has been found that a neutral atmosphere such as is provided with nitrogen, argon, helium or mixtures of various gases, is necessary, with such atmospheres, the annealing temperature may be selected from the range between 1025 C. and 1100 C. The annealing time is not critical, one to four hours, preferably two hours, being a convenient time.

Alternative to sintering and annealing in one firing, the bodies herein may be prepared by sintering in a neutral atmosphere and finally cooling again to room temperature. By this procedure, magnetic cores with similar characteristics to thecore of the example are produced.

Tables I, III, IV, and V tabulate various bodies or cores together with their respective compositions, sintering temperature, squareness ratio (R and coercive force (H in oersteds. Table II tabulates various additional characteristics of the cores of Table I. In Table II, T is the peaking time, T is the switching time, I is the Write current, I is the disturb current which usually equals /2 I uV is the undisturbed 1 output signal and dV is the disturbed 0 output signal. All times are in micro seconds, all currents are in milliamperes, and all signals are in millivolts. The data of the tables were taken with a current pulse rise time of 0.2 microsecond and a pulse duration of 6 microseconds as shown by the curve 35 of FIGURE 3.

Tables I and II set forth, in tabular form, additional examples of cores prepared by the method of the example, but differing from each other only in composition. Composition 9, which contains no added zinc or cadmium is included for comparison purposes. It will be noted that the cores of the invention exhibit a higher squareness ratio (R shorter peaking time (T and shorter switching time (T In addition, the cores of the invention may be prepared in any of a wide range of coercive force, which makes it possible, for example, to tailor the core to a particular transistor or vacuum tube drive. Further, many of the coresof the invention exhibit higher response .or output signal voltage (uV Table III tabulates additional examples of the invention with differing compositions containing 10 to 14 mol percent of ZnO plus CdG. The examples of Table 111 were sintered at the indicated temperatures for about tenhours to obtain comparable values of coercive force, in the range of high drive cores. In each case, the cores of the inven tion which contain both ZnO and CdO exhibit a higher squareness ratio than the core containing only Z110.

Table IV tabulates additional examples similar to those in Table III except that the exampleswere sintered at the indicated temperatures to obtain values of coercive force in the tangent low drive cores. Again, cores containing both ZnO and Cd O (except No. 31) exhibit a higher squareness ration than the core containing only 2110.

Table V tabulates additional examples of theinvention with differing compositions containing mol percent of ZnO plus CdO. The examples of Table V were prepared by the method of the first example. In each case, the 'squareness ratio and the coercive force of the cores containing ZnO and CdO are higher than those of the core (No. '32) which contains only ZnO.

Tables III, IV, and V considered together show that the coercive force of the cores of the invention may be tailored by adjustment of the sintering temperature, without compromise of the squareness ratio. Some other generalizations whch may be made are as follows. The squareness ratio decreases as the molar proportion of CdO approaches that of ZnO. The proportion of ZnO should not be greater than 24 mol percent, preferably in range 5 to 14 mol percent. Also, the molar proportion of Z110 should be greater than that of the CdO.

There have been described ferromagnetic ferrite bodies having substantially square hysteresis characteristics which'provide an output voltage and a relatively fast switching time. There have also been described methods for preparing the ferromagnetic bodies of the invention.

- Table I lComposition, moi percent] 4 Sintoring No. F0103 M110 Mp0 ZnO CdO 121211., R, 11.,

40 v 22 11 a 1,200 .87 1.31 10 22 r .3 1,225 .00 1.10 40 22 8 a 1,200 00 .051 40 22 8 5 1,200 .119 .71 40 22 8 7 1. 200 s: 702 H40 22 11 1 1,100 .53 1.15 40 22 r 5 1,150 .87 1.41 40 22 0 0 1,150 .83 1.44 40 22 a 0 0 1,250 .82 1.30

Table II No. In, 1.1, 11,171 dV, TD, 1.,

ma. H111:- #SQC #5120.

Table III lCoinpositi'on, r1101 percent] Sinteriug No. F010; MnO MgO ZnO CdO- Tgm p R, H,

40 22 27 11 1,115 .75 1.45 40 22 20 6 a 1,150 .83 1.44 40 22 28 r a 1.200 .85 1.44 40 22 20 7 5 1,150 .87 1. 41 40 22 25 r 0 1.150 .83 1.30 40 22 20 a 4 1,150 83' 1. 49 40 22- 24 s 0 1,150 .81 1.48 40 22 27 9 2 1, 200 .83 1. 39 40 22 20 9 a 1,175 .82 1. 50 40 22 25 10 3 1,175 .80 1.40 40 22 20 11 1 1,165 .81- 1. -11

Table l V [Umnposltiom ulol percent] Siuturing N0. 1110; MnO Mg 0 Z110 C110 T enelpq R, H1,

40 22 27 11 1,140 .25 1.04 40 22 26 6 6 1,175 .88 1. 01 4-0 22 28 7 3 1, 250 9t) 1. 07 41) 22 26 7 5 1, 175 .90 1.01 40 22 23 7. 5 7. 5 1, 210 .80 1. O7 40 22 26 8 4 1,175 .91 1.05 +10 22 24 8 6 1,175 87 1. 0.) 40 22 23 8 7 1, 175 85 1. 02 40 22 25 9 4 1,175 9t) 1. 08 40 22 23 t) 6 1,175 87 1.03 40 22 20. 5 12 5. 5 1, 140 78 1. ()9

Table V lcomposition, mol percent] I I i Sintering No. FeaO; M110 MgO ZnO CdO 'lgnn, R. H

1 v i i MnO 20 to 25 2110 5 to 25 Cd() 1 to 12 Fe O 35 to 42 and the combined molar percent of ZnO plus CdO is at least 10 mol percent and the molar proportion of ZnO is greater than the molar proportion of CdO- but not more than 14 times the molar proportion of CdO.

2. A ferromagnetic ferrite body having a substantially rectangular magnetic hysteresis loop and formed by sintering a mixture consisting essentially of magnesium, manganese, zinc, cadmium, and iron oxides in the pro portions of about:

I M01 percent MgO -L 20 to 30 MnO 20' to 25 Zn'O .5 to 14 CdO 1 to 7.5 F3203 35 to 42 -ter'ing a mixture consisting essentially of magnesium,

I manganese, zinc, cadmium, and iron oxides in the proportions of about:

M01 percent 'MgO 10 to 30 MnO 22 Z 5 to 14 CdO -2 1 to 7.5

and the combined molar percent of ZnO plus CdO is between 10 and 15 .mol percent and the molar proportion of ZnO is greater than the molar proportion of CdO.

4. A ferromagnetic ferrite body having a substantially rectangular magnetic'hyster'esis loop and formed by sin- -tering a mixture consisting essentially of magnesium, manganese, zinc,- cadmium, and iron oxides in the proportions of about:

i Mol percent MnO 22 ZnO v 9 CdO 3 F ezog 5. A ferromagnetic ferrite body having a substantially rectangular magnetic hysteresisloop and formed by sin- 1 tering a mixture consisting essentially of magnesium, manganese, zinc, cadmium, and iron oxides in the proportions of about:

. Mol percent MgO 26 MnO 22 .ZnO 7 CdO F6203 6. A ferromagnetic ferrite body having a substantially rectangular magnetic hysteresis loop and formed by sintering a mixture consisting essentially of magnesium, manganese, zinc, cadmium, and iron oxides in the proportions of about:

Mol percent MgO 28 MnOv 22 ZnO 7 Cd() 3 F2O3 7. A ferromagnetic ferrite body having a substantially rectangular magnetic hysteresis loop and formed by sintering a mixture consisting essentially of magnesium, manganese, zinc, cadmium, and iron oxides in the proportions of about:

Moi percent MgO 23 MnO 22 ZnO 12 CdO 3 F6203 4O 8. A process for preparing a ferromagnetic ferrite body having a substantially rectangular magnetic hysteresis loop comprising calcining an intimate physical mixture consisting essentially of:

Mol percent MgO to 30 MnO 20 to 25 ZnO 5 to 25 CdO 1 to 12 Fezog t0 and the combined molar percent of ZnO plus CdO isat least 10 mol percent and ZnO is present in greater proportion than CdO but not more than 14 times the molar proportion of CdO, compacting said calcine to a coherent body, and then sintering said compacted body at about l100 to1300 C.

, 9. A process for preparing a ferromagnetic ferrite body having a substantially rectangular magnetic hysteresis loop comprising calcining at about 900 to 1100 C. and

intimate physical mixture consisting essentially of:

' A Mol percent MgO 10 to 30 MnO to ZnO 5 to 14 CdO 1 to 7.5 Fe O 35 to 42 and the combined molar percent of ZnO plus CdO is between 10 and 15 mol percent and the molar proportion of ZnO is greater than the molar proportion of CdO,

compacting said calcine to a coherent body, sin-tering said compacted body at about 1100 to 1300 C. in air, and annealing said sintered body at about 1025 to 1100 C. in a neutral atmosphere.

10. The process of claim 9 wherein said neutral atmosphere is nitrogen.

11. A process for preparing a mixed ferrospinel having asubstantially rectangular magnetic hysteresis loop comprising calcining at about 900 to 1100 C. in intimate physical mixture consisting essentially of:

' .Mol percent MgO- -4 10 to MnO 20 to 25 ZnO 5 to 14 CdO 1 to 7.5 Fe O to 42 compacting said calcine to a coherent body sintering said compacted body at about 1100 to 1300 C. in air, cooling said sintered body to room temperature and then annealing said body in a neutral atmosphere.

12. The process of claim 11 wherein said neutral atmosphere is nitrogen. 

1. A FERROMAGNETIC FERRITE BODY HAVING A SUBSTANTIALLY RECTANGULAR MAGNETIC HYSTERESIS LOOP AND FORMED BY SINTERING A MIXTURE CONSISTING ESSENTIALLY OF MAGNESIUM, MANGANESE, ZINC, CADMIUM, AND IRON OXIDES IN THE PROPORTIONS OF ABOUT: 